The present invention relates to a device for stirring and aerating a liquid and eliminating foam in a tank or vat treating this liquid, such as for example fatty waste products extracted from effluents of town sewage or agricultural food industries.
As is known, the town sewage effluents which circulate in discharging pipes such as drains are collected in purification stations in which various treating operations are carried out to purify them before returning them to the natural medium.
This purification produces by-products related to the purification such as fats, sludges.
Dumping sites are less and less inclined to accept this type of products so that the purification stations must now be capable of treating them and limiting them as far as possible.
Thus, the fats trapped at the inlet of the purification station are poured into a treating apparatus such as for example a biological decomposition tank which comprises a mechanical stirrer for stirring the liquid.
It is known from European patent application No. 0 687 497 that such a decomposition tank may be of the type comprising:
a vertical sheath extending in a part of the height of the tank and comprising, in its upper part, at least a first opening providing an entry passage for the liquid and, near the bottom of this tank, at least a second opening providing an exit passage for the liquid,
a funnel located around the upper part of the sheath and connected to said sheath, the upper edge of said funnel defining with the wall of the tank a free space for the passage of the liquid,
a main propeller located inside the sheath below the bottom of the funnel,
a vertical shaft driven in rotation, extending inside the sheath above this propeller and driving the latter in rotation,
an anti-vortex means located inside the sheath, below said propeller,
means for injecting a gas under pressure in the sheath, below the antivortex means, and
dynamic stirring means for stirring the liquid inside the sheath, these means being located below the anti-vortex means.
In this prior art apparatus, said dynamic stirring means comprise a prolonged section of said shaft which extends from said propeller down to a bearing which is fastened to the bottom of the tank.
Rods are fastened along said section of the shaft so as to stir dynamically the liquid which is located inside the sheath when the shaft rotates.
For mechanical reasons, which are partly due to the fact that the shaft has a tendency to vibrate, the latter cannot be very long. A length of about 15 feet is typical. Thereby, the depth of the tank is also typically limited to about 15 feet.
This restriction is too bad, since the efficiency of such an apparatus increases with the depth of the tank. In fact, due to the law of Henry, the deeper the thank, the greater the solubility of the gas which is introduced under pressure in the sheath, and thereby the greater the biological activity inside the liquid and the faster the decomposition of the products comprised in this liquid.
Moreover, due to the friction which is exerted by the liquid on the rods, the power transmitted by the driving means to the shaft has to be very important.
Further, stringy materials frequently get tangled with the rods, thereby reducing the efficiency of the stirring. One must then stop the apparatus and pull the shaft out of the tank in order to remove the stringy materials.
Besides, after a certain time of use, one also has to replace the bearing located at the bottom of the tank and/or the shaft. For doing so, the whole apparatus has to be stopped and the tank has to be emptied, which is both complicated and time consuming.
It is therefore an object of the present invention to avoid these drawbacks by proposing a device for stirring and aerating a liquid, such as for example fatty waste products extracted from town sewage effluents or agricultural food industries, which can be used in very deep tanks, which consumes less energy than the aforesaid apparatus, which remains efficient despite the presence of stringy materials, and which requires very little maintenance.
Further, in the course of the treatment of the liquid, such as for example fatty waste products, there is formed on the surface of the liquid a layer of foam which tends to fly away and to pollute the environment.
It is therefore necessary to provide means for eliminating this foam floating on the surface of the liquid in the course of the treatment of the latter.
Up to the present time, these means have been formed by stirrers located at the level of the surface of the liquid and adapted to flatten the foam.
Yet, these stirrers are not entirely satisfactory and a part of the foam flies away and pollutes the environment.
It is therefore an other object of the present invention to provide more efficient means for recovering the foam floating on the surface of the liquid.
The first aforesaid object is reached with a device for stirring and aerating a liquid in a tank for treating said liquid, comprising inside said tank:
a vertical sheath extending in a part of the height of said tank and comprising, in its upper part, at least a first opening providing an entry passage for said liquid and, near the bottom of said tank, at least a second opening providing an exit passage for said liquid,
a funnel located around the upper part of said sheath and connected to said sheath, the upper edge of said funnel defining with a wall of said tank a free space for the passage of the liquid,
a main propeller located inside said sheath below the bottom of said funnel,
a vertical shaft driven in rotation, extending inside said sheath above said propeller and driving the latter in rotation,
an anti-vortex means located inside said sheath, below said propeller,
means for injecting a gas under pressure in said sheath, below said anti-vortex means, and
uniquely static stirring means for stirring said liquid by turbulence inside said sheath, said static stirring means being located below said antivortex means.
The second aforesaid object is reached with means for eliminating foam in said tank, comprising:
a first series of ports provided in the upper part of said sheath and located at the level of said liquid in said funnel,
a second series of ports provided in said sheath, above the bottom of said funnel, and
means for regulating the level of said liquid in said funnel.
Further features and advantages of the invention will be apparent in the course of the following description with reference to the accompanying drawings.